The cellular proto-oncogene c-myc has been independently captured by avian retroviruses several times to generate acutely transforming viruses. We are studying two such retroviruses, MC29 and FH3, in which myc is expressed as a fusion protein with the retroviral gag gene. Both MC29 and FH2 transform avian macrophages in culture, but only MC29 can transform fibroblasts. Our results indicate that the Gag component of the FH3 original isolate (FH3E) down capable of transforming fibroblasts; several of these have suffered deletions in gag leading to smaller fusion proteins. These viruses will be utilized in order to understand the role of the gag sequences in regulation of transformation. We have also constructed an MC29 mutant lacking the nuclear localization signal (NLS). We have used this mutant to show that very low levels of MC29 Gag-Myc in the nucleus (equivalent to the level of endogenous c-myc expression. Thus it is the small fraction of Gag-Myc in the nucleus which can bind to DNA which is responsible for transformation. We propose to use a variety of chimeric viral constructs with differing Gag portions and Myc mutations in order to delineate the mechanism of v-myc enduced single- hit transformation of avian cells i culture. We propose four specific aims: Determine which regions of FH3E Gag are responsible for the failure to transform fibroblasts. Compare the biochemical properties of Gag-Myc proteins which differ in their abilities to transform fibroblasts. Examine the effect of mutations in Myc nuclear localization sequence and basic (DNA binding) region of macrophage and fibroblast transformation . Determine whether down-modulation of c-myc expression is a characteristic feature of v-myc induced transformation. Determine whether some of our viral constructs can be used to develop an optimal system to find genes regulated by v-Gag-Myc.